Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. The various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, and polyvinylidene fluoride polymers (PVDF). Commercially employed fluoropolymers also include fluoroelastomers and thermoplastic fluoropolymers.
Several methods are known to produce fluoropolymers. Such methods include suspension polymerization, aqueous emulsion polymerization, solution polymerization, polymerization using supercritical CO2, and polymerization in the gas phase.
Currently, the most commonly employed polymerization methods include suspension polymerization and aqueous emulsion polymerization. The aqueous emulsion polymerization normally involves polymerization in the presence of a fluorinated emulsifier, which is generally used for stabilization of the polymer particles formed. The suspension polymerization generally does not involve the use of surfactant but results in substantially larger polymer particles than when aqueous emulsion polymerization is used. Thus, polymer particles created using suspension polymerization will quickly settle out whereas polymer particles created using dispersions obtained in emulsion polymerization generally provide good stability over a long period of time.
An aqueous emulsion polymerization where no surfactant is used, or emulsifier free polymerization, is known to generally produce homo- and copolymers of chlorotrifluoroethylene (CTFE). In one such emulsifier free aqueous polymerization, a radical initiator system of a reducing agent and oxidizing agent is used to initiate the polymerization, where the initiator system is added in one or more further charges during the polymerization. These types of emulsifier free polymerizations are also known.
Notwithstanding the fact that emulsifier free polymerizations are known, a process using aqueous emulsion polymerization in the presence of fluorinated emulsifiers is still a desirable process to produce fluoropolymers because it results in stable fluoropolymer particle dispersions in high yield. Frequently, the emulsion polymerization process is carried out using a perfluoroalkanoic acid or salt thereof as an emulsifier. These emulsifiers are typically used because they provide a wide variety of desirable properties, such as high speed of polymerization, good copolymerization properties of fluorinated olefins with comonomers, small particle sizes of the resulting dispersion, desirable stability, and good polymerization yields, i.e. a high amount of solids can be produced. However, environmental concerns have been raised with regard to using these types of emulsifiers. In particular, perfluorinated alkanoic acids having 8 or more carbon atoms and their salts, which have been hereto a preferred class of perfluorinated emulsifiers, are now known to be bio-accumulating. Moreover, these emulsifiers are generally expensive.
Alternative emulsifiers to the perfluoroalkanoic acids or salts thereof have been proposed in the art for conducting the emulsion polymerization of fluorinated monomers. For example, emulsifiers of the general formula Rf—C2H4—SO3M, wherein Rf represents a perfluorinated aliphatic group and wherein M represents a cation, have been disclosed. Other exemplary partially fluorinated emulsifiers of the general formula Rf—(CH2)m-R′f-COOM have been disclosed, where Rf represents a perfluoroalkyl group or a perfluoroalkoxy group of 3 to 8 carbon atoms, R′f represents a perfluoroalkylene of 1 to 4 carbon atoms and m is 1-3. Perfluoroalkoxy benzene sulfonic acids and salts thereof have been disclosed in the aqueous emulsion polymerization of fluorinated monomers.
Functionalized perfluoropolyethers of the general formula: F—(CF2)m-O—[CFX—CF2—O]n-CFX—COOA are known, where m is 1 to 5, X is F or CF3, A is a monovalent cation and n is 0 to 10. In particular, perfluoropolyether acids are taught as emulsifiers in the emulsion polymerization of ethylenically unsaturated monomers. Other known fluorinated polyethers include those having the formula:
F—(CF2)m-O—[CFX—CF2—O]n-CFX—COOA wherein m is 3 to 10, X is F or a perfluoroalkyl group, n is 0, 1 or 2 and A is the counter ion of the carboxylic anion. These functionalized polyethers are taught as emulsifiers in the emulsion polymerization of fluorinated olefins. Fluorinated emulsifiers of the formula: C2F5O(CF2CF2O)mCF2COOA, where A is a hydrogen atom, an alkali metal or NH4+, and m is an integer from 1 to 3 are also known as alternative emulsifiers.
The use of perfluoropolyethers as co-emulsifiers, or emulsifiers that require the presence of other emulsifiers, such as perfluoropolyether acids in an aqueous emulsion polymerization is known. For example, the use of microemulsion prepared from perfluoropolyethers having neutral end groups in an aqueous emulsion polymerization of fluorinated monomers has been disclosed. It is known that certain perfluoropolyethers having carboxylic acid groups or salts thereof at both end groups, i.e. the perfluoropolyethers are bifunctional. The functionalized and/or unfunctionalized perfluoropolyethers are taught for use in aqueous dispersions of fluoropolymers and in the preparation of such dispersion by aqueous emulsion polymerization. The use of a combination of perfluoropolyether surfactants having a carboxylic acid group or salt thereof with a fluoroalkyl carboxylic acid or sulfonic acid or salt thereof are also known. It is taught that the perfluoropolyether surfactants on their own are not very powerful surfactants. Fluorinated ether surfactants for emulsion polymerization and oligomers from VDF are also known for emulsion polymerization.
While there have been advances in alternative emulsifiers, fluoroelastomers derived from known alternative emulsifiers have metal contents that are too high for some applications. In particular, fluoroelastomers can be used in the semiconductor industry in microchip manufacturing where the fluoroelastomer may be used in seats of microchip fabrication equipment. During microchip manufacturing, the fluoroelastomer can be exposed to high temperature and aggressive chemicals. Besides resistance to aggressive chemical and/or heat, it is also desirable for fluoroelastomers to have low glass transition temperatures, which makes them suitable for use at low temperatures as is required, for example, in applications in automobiles or aircraft industries. While fluoroelastomers, and in particular perfluoroelastomers are already being used in the semiconductor industry, there continues to be a need to modify the polymerization of these fluoroelastomers to eliminate the use of perfluoroalkanoic acid or salt thereof so as to make them more suitable for this specialized application. In particular, there is a need to modify the polymerization such that fluoroelastomers having a low metal content result. Specifically, there is a need for an alternative emulsifier that does not create fluoroelastomer compositions having high amounts of metal cations, which limits their suitability for use in the microchip manufacturing. Desirably, the alternative emulsifier is biodegradable and has a low retention time in the human body.
The present disclosure is directed to an alternative fluorinated emulsifier that provides improved quality, for example lower metal content and good compression set, of resulting elastomers, in particular those having a low glass transition temperature. Low metal content in resulting elastomers is important in semi-conductor applications where the resulting elastomers are used as sealing materials. In another aspect of the present disclosure, alternative fluorinated emulsifier concentrations in the resulting elastomers are relatively low, which results in beneficial effects on cure chemistry for articles derived from these elastomers.